The present invention relates to a method and an apparatus for signal transmission suitable for use in a system capable of transmitting and receiving a signal at a plurality of signal transmission speeds in accordance with an IEEE1394 standard, for example, and a signal transmission system.
Conventionally, with respect to signal transmission speed of an optical fiber two-way communication system, for example, capable of transmitting a signal at a plurality of signal transmission speeds, consideration has been primarily given only to speeds handled by a transmitter and a receiver connected to both ends of an optical fiber, while an optical fiber having a characteristic of higher-speed transmission than the speeds handled by the transmitter and receiver connected to both ends of the optical fiber has been used.
According to a method of determining a transmission speed by a transmission protocol defined in an IEEE (Institute of Electrical and Electronics Engineers) 1394. a draft, as an example of a method of determining signal transmission speed in consideration of the speeds handled by the transmitter and receiver connected to both ends of the optical fiber, each of the transmitter and receiver connected to both ends of the optical fiber compares its maximum transmission speed, or a capacity of the apparatus, with a maximum transmission speed of the other apparatus, to determine the lower maximum transmission speed as the maximum communication speed to be shared between the apparatus.
The maximum transmission speed shared between the optical transmitter and receiver connected to both ends of the optical fiber in the system transmitting and receiving a signal using the optical fiber as described above is determined by capacity of a light source such as a laser diode (LD) or a light emitting diode (LED) and capacity of a circuit for driving the light source in the case of the optical transmitter. In the case of the optical receiver, on the other hand, the maximum transmission speed is determined by capacity of a light receiving unit such as a photodetector (PD), a circuit system connected to the light receiving unit, and the like. The optical fiber is used as a transmission line between the optical transmitter and receiver on condition that the optical fiber has a characteristic of higher-speed transmission than the speeds handled by the transmitter and receiver connected to both ends of the optical fiber, and has a length within a range allowing normal transmission of signals.
According to the IEEE1394 standard, transmission is made basically by using two pairs of twisted-pair lines, and the transmission method is so-called half-duplex communication using both the two pairs of twisted-pair lines for one-way transmission. This communication method uses a communication technique referred to as DS coding, which sends data in one pair of the twisted-pair lines and a signal referred to as strobe in the other pair of the twisted-pair lines, and then reproduces a clock on the receiving side by obtaining an exclusive disjunction of the two signals. The IEEE1394 standard currently defines three speeds: 98.304 Mbps (S100), 196.608 Mbps (S200), and 393.216 Mbps (S400) as maximum transmission speed (data rate), and specifies so-called upward compatibility, which means that an apparatus capable of a high rate needs to support an apparatus (node) having a lower rate.
Recently, there have been cases of putting an optical fiber compliant with the IEEE1394 standard to practical use, in which an AV apparatus, a personal computer and the like for a general household, for example, are networked by using the optical fiber compliant with the IEEE1394 standard. Therefore, it is to be expected that there will be increasing uses hereafter in which various household apparatus are networked and thus a general consumer (general user) himself/herself connects the various apparatus freely and arbitrarily by using an optical fiber. Furthermore, with the networking of such household apparatus, various optical fibers are expected to be commercialized hereafter.
However, when a general user, or a non-engineer, connects the various apparatus freely and arbitrarily by using the various optical fibers, as described above, it is quite conceivable that actual signal communication cannot be carried out even if the apparatus are physically connectable.
Specifically, in general, when constructing a communication network system, an engineer constructs the network after adequate consideration of characteristics of a transmission line and various apparatus of the network system. However, when a general user connects the various household apparatus freely and arbitrarily by using the various optical fibers, as described above, an optical fiber other than optical fibers assumed by a designer of each of the apparatus at the time of design may be used, or a combination of optical fibers not assumed by the designer at the time of design may be used, for example. The use of the optical fibers not assumed by the designer may result in impossibility of communication.
More specifically, it is quite conceivable that when a multimode optical fiber longer than was assumed by the designer at the time of design is used, for example, or an optical fiber formed by combining a plurality of multimode optical fibers with each other, for example, not assumed at the time of design is used, a signal may not be transmitted or received because of a characteristic mismatch between the apparatus and the optical fiber, even if communication is possible in principle. Specifically, when a relatively long multimode optical fiber is used or a plurality of optical fibers are combined with each other, or especially when a plurality of multimode optical fibers are combined with each other, not only does transmission loss become greater, but a communication speed enabling transmission is also limited by mode dispersion to a lower level than when a short optical fiber is used. A bandwidth (B) of a step index fiber is expressed by the following expression:B∝c/(nΔL)
where n is an index of refraction; Δ is dispersion; c is the speed of light; and L is length of the fiber. This expression indicates that as the length L of the fiber is increased, the transmission bandwidth B is decreased. The above is described in literature such for example as “New Edition Optical Fiber Transmission: Noda et al., the Institute of Electronics and Communication Engineers of Japan (Corona Publishing), p58 (1984),” “An Introduction to Optical Fiber Communication: Suematsu et al., Ohmsha, p213 (1989) ISDN 4-274-03266-3,” and “Plastic Optical Fiber: POF Consortium (Kyoritsu Shuppan), p23 (1997) ISDN 4-320-08545-0.”
In addition, because of the above, the system will be user-friendlier if notifying the user that communication is not possible when actual signal communication cannot be carried out.